This invention relates to apparatus, which handle solids, and more particularly to such apparatus useful for dispersing solids into liquids.
Mixers are well known in the art. Mixers have been used to mix solids with other solids and solids with liquids. Solids, as used herein, refers to particulate materials having a median particle size ranging from about 1 micron to about 2 centimeters. Typically solids used with the present invention will have a median particle size ranging from about 20 to 500 microns. Median particle size is measured according to ASTM Standard E1638, incorporated herein by reference. Liquids refers to incompressible materials having no shear modulus. It is to be understood that a mixer may have one or more solids and one or more liquids. The invention described and claimed herein is equally well suited for single and plural solid and/or liquid combinations.
The solids are typically introduced to the mixer through a series of stages in an apparatus. The mixer may be one stage at an intermediate position in or near the end of the apparatus. The first stage of the apparatus is typically a hopper. Solids are introduced to the hopper from a bulk raw material supply. Optionally the hopper may have agitation to assist in transfer of the solids from the hopper. The solids are often transferred through different stages of the apparatus using one or more augers. As used herein an auger is an axially rotatable screw feed. The auger may ultimately feed the solids into a mixer. One or more liquids may be added to the mixer. The mixer has an axially rotatable impeller for dispersing one or more solids throughout the liquid(s). The impeller may create a vacuum in the mixer, as an artifact of the centrifugal mixing process. The solid/liquid dispersion may be drained or pumped from the mixer. The dispersion may be used as a premix for yet another batch or continuous process or may be used as an end product.
It is typically important that the solids be thoroughly and uniformly dispersed throughout the liquid. Properties inherent to the solids may make proper dispersion more difficult to obtain. For example, as particle size decreases and cohesion and the propensity of the solids to hydrate increases, proper dispersion becomes more difficult. Likewise, properties inherent to the liquid may make proper dispersion more difficult to obtain. For example, as viscosity, temperature and backpressure at the mixer outlet increase, proper dispersion becomes more difficult.
Likewise, properties inherent to the apparatus may make proper dispersion of the solids into the liquid more difficult to obtain. For example the vacuum in the mixer may draw solids at an uncontrolled delivery rate. Instead of a constant supply rate, the solids may be supplied to the mixer at a variable supply rate. The variable supply rate may provide more solids at one point in time than can be dispersed by the impeller and less solids at a different point in time. While the impeller imparts a uniform shear rate at any radial position, differences in the amount of solids present may make uniform dispersion more difficult to obtain.
One example of a prior art apparatus is found in U.S. Pat. No. 5,547,276 issued Aug. 20, 1996 to Sulzbach et al. The Sulzbach et al. apparatus transfers solids from a storage vessel to an intermediate tank via a horizontally oriented screw. The solids are transferred from the intermediate tank to a mixing apparatus via a second horizontally oriented screw. Sulzbach et al. also shows a complex arrangement having a vacuum pump and a feedback control device deareates the solids in the intermediate tank. This complex arrangement increases the cost of the Sulzbach et al. apparatus. Furthermore, the horizontally oriented screw increases the apparatus"" footprint, increasing the operating cost due to the floor space requirements.
An example of the introduction of particulate material into a receiver is found in U.S. Pat. No. 6,021,821 issued Feb. 8, 2000 to Wegman. Wegman uses a vertically oriented auger to feed fluidized particulate material into a receiver. The receiver has a negative pressure, due to a vacuum assist of up to 10 inches (25.4 cm) of water. Wegman does not teach handling of particulate material under high differential pressure conditions, as often occurs when mixing solids and liquids together. Nor does Wegman teach how to handle materials, such as anthracite coal, or maltodextrin, which become floodable when subjected to fluidization.
The present invention provides an apparatus and method for achieving a controlled delivery rate of solids into a mixer, without the need for a deareating or evacuation step. The present invention also provides an apparatus and method for achieving controlled delivery of solids into a mixer for dispersion throughout one or more liquids or gasses.